You may want to look at the xorg-server build log if anything is wrong there. Set FEATURES="keeptemp" and rebuild the package, then retrieve the log from /var/tmp/portage/x11-base/xorg-server/temp/build.log

This bug suggested to me that suid set the --enable-install-setuid configure flag. Line 23 of my build.log, however does not have --enable-install-setuid. Little surprise then that the SUID bit on my /use/bin/Xorg ends up not set.

Some fstab documentation suggests defaults should be the option for ext4, which is what I'm on. That was the problem, previously it was noatime. Now, with defaults, the SUID bit is preserved when copied.

That's weird: I've always had to turn suid off with nosuid, so my /usr partition has just 'noatime' as well, and Xorg is properly suid. What version of sys-apps/util-linux are you on? (That provides the /bin/mount binary.) I'm on 2.21.2 (stable.) Perhaps something has changed recently.

edit: ah I'm on xorg-server-1.13-0-r1 still. I held off the upgrade as I wanted to get update to trigger a rebuild of xorg-drivers generically, instead of having to flag every big upgrade in /etc/warning. Then I thought perhaps portages sub-slots might make it unnecessary. Still, it's odd that prior mount shouldn't have had an issue.